The old wives' tale turns out to be true: lying with your feet up after sex improves your chances of having a baby (if you're a woman, that is).

True, at least, for a specialised kind of sex – that involving artificial insemination, where the sperm is directly inserted into the womb using the medical equivalent of a turkey baster.

Researchers from the Netherlands, who followed 400 couples who had the treatment, found that those who lay on their backs for 15 minutes afterwards were 50 per cent more likely to conceive than those who got up and walked about immediately. The pregnancy rate was 27 per cent among those who "lay back and thought of babies" but only 18 per cent among those who moved around.

Asked if advice to lie still should be offered to couples trying for a baby the natural way, Dr Inge Custers of the Academic Medical Centre in Amsterdam, who led the study published in the British Medical Journal, said: "The mechanism of intrauterine insemination is of course quite different from normal sex. Since we have not investigated the latter, your question, which is of course an interesting one, cannot be answered by our study. Remaining in bed after sex, however, is unlikely to influence pregnancy in a negative way."

Professor William Ledger, head of reproductive and developmental medicine at the University of Sheffield, went further. In a commentary published alongside the paper in the BMJ, he said: "The findings agree with the intuitive idea that lying with a 'feet-up' tilt for a few minutes after insemination, either after intercourse or intrauterine insemination, allows the sperm to ascend into the uterine cavity, before standing up brings the negative influences of gravity into play."

However, Professor Ledger added that lying still after intercourse was advocated in the US many years ago – but it "did not seem to improve conception rates after sex".

Artificial insemination is one of the commonest treatments for infertility and is often the first resort of couples because it is simple and cheap, and can be easily repeated. Normally drugs are given to stimulate the ovaries before insemination but it can also be performed drug-free. The procedure is recommended by the National Institute for Clinical Excellence.

Success rates vary widely, however: from 5 per cent to 70 per cent. Dr Custers and colleagues say it is not clear why lying still after the procedure improved success rates in their study but, like Professor ledger, they speculate that moving around immediately may cause "leakage".

They argue that all women having intrauterine insemination should be able to lie down for 15 minutes afterwards. "Although immobilisation takes more time and occupies more space in busy rooms, the intervention will be economic in the long run as pregnant patients will not return in subsequent cycles."

Professor Ledger said giving women an extra 15 minutes per appointment in busy clinics will put pressure on them although "with planning this would not be insurmountable". But before going down that road he suggested clinics should carry out their own trials to test the results "in the real world".

"If successful, more couples could be spared the rigorous and costly process of IVF. Future trials should assess the effect of different durations of immobilisation."